1. Field of the Invention
The present invention relates to a semiconductor integrated circuit EEPROM. More particularly, the present invention relates to an EEPROM circuit and the design thereof having security functions. The EEPROM circuit disclosed in the invention is provided with unique security functions. In particular, security functions can be assigned for a specific byte, word, a portion of a byte, or a portion of a word. Furthermore, the present invention provides security configurations for an EEPROM which can be incorporated in small spaces.
2. Background Art
EEPROM (Electrically Erasable Programmable ROM) is a kind of non-volatile memory usually utilized in circuit design. The content of memory may be kept automatically even when a power supply for the EEPROM is switched off abruptly. Generally, EEPROM is used for storing important information, like configuration or password data. As for other applications, for example, read-only memory in which a "write" function is unnecessary or prohibited, the corresponding EEPROM must be set to be under "security" mode. Furthermore, for some systems the content of memory may be re-written because of erroneous operations. In general, while switching on the power supply of a system in which EEPROM is included, different power-on reset voltages may be set for each chip respectively. Under the circumstances, the input ends of the corresponding EEPROM may receive undesirable noises and lead to erroneous operations, i.e. executing a "write" operation. Erroneous data are therefore stored in the EEPROM.
To solve this problem, it has been proposed to establish, in corresponding EEPROMs, separate security circuits which can provide security for all memory cells of the EEPROMs. However, since such a security circuit can provide security only for all or none of the memory cells, it apparently does not satisfy general requirements in application. If it is desirable to provide individual security only for a portion of memory or for each memory cell by the afore-described technique, the corresponding circuit design may become very complicated, thus leading to high cost and significant chip area.
Therefore, it is necessary to provide a security configuration for EEPROM memory circuits by which security functions for any specific memory cells are possible. It is also necessary to provide a simple and compact security configuration for specific memory cells or bytes of an EEPROM.